Add type_as attribute to specta

[mablin7]

We were looking into using specta in our project, but ran into the issue of needing an escape hatch for certain types that can't (easily) implement Type. This solution is similar to ts_rs's #[ts(type = "..")] attribute.

#[specta(type_as = <Ident>)] on a field overrides its type in the generated type definition with the type of the identifier.

Example:

use specta::{ts::ts_export, Type};

pub enum MyEnum {
    A,
    B,
    C,
}

#[derive(Type)]
pub struct MyCustomType {
    #[specta(type_as=String)]
    pub nested: MyEnum,
}

fn main() {
    println!("{}", ts_export::<MyCustomType>().unwrap(),);
}

Any feedback is super appreciated!

[Brendonovich]

Thanks for working on this! Super cool idea to override the Rust type that is used rather than writing custom typescript.
I'd like the feature to mirror ts-rs a bit more, but you've already done quite a good job.
I might rebase this PR onto a bunch of new changes to Specta that are being made in preparation to make it more of a standalone library, I'll handle that though.

[Brendonovich]

Also would you mind explaining why you need type overrides, and why you picked Specta over ts-rs? Just curious haha

[oscartbeaumont]

Thanks for your time. Looks great!

[mablin7]

Also would you mind explaining why you need type overrides, and why you picked Specta over ts-rs? Just curious haha

Oh, sorry I didn't have the time to address your comments, thanks for the corrections.

Our main problem was that the messages in our tauri app included types from 3rd party crates that we normally just serialize to string, but we can't have them implement Type.

The reason we would like to use specta is its ability to generate all referenced type definitions recursively. With ts-rs we have a huge list of types that need to be printed, and it needs to be updated manually. Whereas with specta, it would just be a single call, since we already have all relevant types in an enum anyway.